If you find a bottle of sunscreen packed in last year's pool bag, here's a suggestion: Toss it out. Since the active compounds can degrade and lose their effectiveness, slathering on old lotion or spray is one mistake people make when trying to protect their skin.

Each year about 84,000 people in the U.S. are diagnosed with melanoma and more than 8,000 die from this type of skin cancer. In addition, millions of cases of basal cell and squamous cell carcinomas are diagnosed each year, and about 90% of these skin cancers are linked to ultraviolet (UV) radiation from the sun.

Using sunscreen plays a "key role" in protecting your skin, according to the American Academy of Dermatology Association. So, we asked dermatologists to share their tips to optimize protection and we learned about the common misperceptions they hear from patients.

1 Concerned about chemicals? Try a mineral alternative

Research shows some of the active ingredients used in chemical sunscreens are absorbed into the bloodstream, and the FDA has been conducting a safety review. The agency says there's "inadequate data" to support a safety finding for some chemicals such as oxybenzone, but there's also no evidence of harm. And most dermatologists say the risk of sunburn likely far outweighs any potential risk from sunscreen chemicals. Still, if you're concerned there are options to avoid these compounds.

Alternatives include physical sunscreens, also called mineral sunscreens, made from zinc oxide and titanium dioxide, which can physically block UV light.

"I think that zinc oxide and titanium dioxide are much safer than chemical sunscreens because they're so inert," says Dr.Tola Oyesanya, a dermatologist with Kaiser Permanente in the Baltimore area. She says mineral sunblocks are better for sensitive skin since they're less likely to irritate.

To avoid the "1980s lifeguard look" with the thick, white paste, there are more zinc oxide products available now that are much clearer, and more "cosmetically elegant," says Dr. Jennifer Holman, a dermatologist and fellow of the American Academy of Dermatology.

Dr. Ida Orengo, chair of the Department of Dermatology at Baylor College of Medicine says, unlike chemical sunscreens, these mineral-based sunblocks can stay on the surface of the skin and "act as a shield or barrier" to deflect sunlight.

And the National Oceanic and Atmospheric Administration says sunscreens without chemicals are better for coral reefs and marine life.

2 Applying enough sunscreen is as important as the SPF

A sunscreen with an SPF, or sun protection factor, of 15 blocks about 93% of UV rays, and when you bump up to SPF 30, you're blocking about 97% of UV rays. Higher than that, "you're not getting a whole lot more sun protection," Dr. Orengo says.

"SPF 30 is sufficient," says Dr. Oyesanya. "There's no sunscreen that filters 100% of the sun's rays, so 97% is pretty good,"she says. Rather than focus on SPF, Oyesanya says pay attention to the amount of sunscreen you apply. Skimping is one of the mistakes many people make.

So, here's a guide: Apply the equivalent of a shot glass which is about 1.5 ounces of liquid sunscreen to cover both your body and face. If you are just covering your face use about a teaspoon.

"Spray sunscreens are a bit risky because it's easy to miss a whole area of your body, especially if you're applying it outside in the wind, Oyesanya says. "Make sure your skin is just totally wet in all the areas that you need to cover," she says, since there isn't an easy way to measure the amount of spray you've applied.

3 Higher SPF sunscreens don't last all day

Many people use SPF 50, or even higher sun protection factor products, and assume it gives them longer lasting protection. "The misconception is that it lasts twice as long. It's not true," says Dr. Gregory Papadeas, a dermatologist in Denver, Colorado.

Even higher SPF sunscreens need frequent reapplication. "You wear them off, especially if you're swimming or sweating," says Dr. Orengo.

Dermatologists recommend that people reapply sunscreen every 2 hours to ensure full protection. Papadeas says, in his household, they buy new sunscreen products each season, since "the chemicals become weaker" and are less effective over time. So, look for the expiration date, or better yet, restock with new products at the beginning of summer.

4 Cloudy days can lead to sunburns

Lots of people assume clouds protect them from the sun and forget to put on sun protection. But it's easy to get sunburned on an overcast day.

Clouds block about 20% of the sunlight, explains Dr. Holman. "You're still getting about 80% of the UV rays filtered through those clouds," Holman says. "You absolutely can still experience damage from UV radiation on a cloudy day." So, even when it's overcast, remember to keep some sunscreen handy.

But it's best not to store your sunscreen in hot places such as the trunk or glove box of your car. "When sunscreen is kept in a hot place, the sunscreen is actually being degraded by heat," says Dr. Oyesanya. It's best to store it in a cool, dry place, to have it last throughout the season.

5 Hats, clothing and sunglasses help block the sun

Baseball caps can protect your forehead, but other parts of your face are exposed. If you want sun protection from a hat, "we always recommend a three inch brim hat that's [made from] a tightly woven material," says Dr. Orengo.

The sun can damage your eyes, as well. According to doctors at Johns Hopkins University, even one day in the sun can burn the cornea, and over time, sun exposure can cause cataracts. The optimal way to protect your eyes is to wear sunglasses that provide 100% UV protection, according to the Hopkins doctors.

There are lots of shirts, hats and other clothing that have sun protection built into the fabric, from brands including Columbia, Cotopaxi, Sombra and Mott50. Dr. Orengo also tells her patients about Sun Guard, a powder you can put into your washing machine to coat your clothes with a chemical sunscreen, giving your washed clothing a SPF of 30, which lasts through multiple washes. And, one more product that may appeal to families with young children: UV stickers that you place on your skin. These stickers change colors when it's time to reapply sunscreen.

6 People of all skin tones and ages benefit from sunscreen

With the exception of babies under 6 months old, sunscreen is recommended for all groups. Although fair-skinned people can be more prone to sunburns, all skin types are vulnerable to damage from the ultraviolet radiation from the sun. "I've cut skin cancers off of every skin type " says Dr. Jennifer Holman.

The risk of melanoma in someone with dark skin is certainly less than someone who has lighter skin, says Dr. Oyesanya, "but it's not impossible." She says people with darker skin who've had a lot of sun exposure should be careful to check their palms, the soles of the feet, nails, inside their mouth, and their toenails. "These are all areas where you can develop skin cancer, and that's because there's less melanin in those areas," she says.